Conventionally, as described in Patent Document 1 and Patent Document 2 for example, optical components such as lenses mounted on optical apparatuses such as optical imaging units have been made of glass, plastic, or single crystals such as lithium niobate (LiNbO3).
Since glass and plastic have high light transmittance and can be easily formed into a desired shape, they are mainly used for optical components such as lenses. On the other hand, single crystals such as LiNbO3 are mainly used for optical components such as light wave guides using the electric-optical properties or birefringence of the single crystals. Optical apparatuses using these optical components, for example an optical pickup, are required to become further miniaturized or thinned.
However, since refractive indices of conventional glasses or plastics are less than 1.9, there is a limitation upon the amount of miniaturization or thinning of the optical components and optical apparatuses using glasses or plastics. Furthermore, since plastics, in particular, not only have disadvantageously low resistance against humidity but also cause birefringence in some cases, plastics also have the problem of a difficulty in efficiently transmitting and condensing incident light.
On the other hand, while single crystals such as LiNbO3 have relatively high refractive indices of as high as 2.3, the single crystals also produce birefringence. Therefore, it is difficult to use the single crystals for optical components such as lenses, and thus the application of the single crystals is disadvantageously limited.
A translucent ceramic including a Ba(Mg, Ta)O3 type perovskite as a main component is disclosed in Patent Document 3 as a material having no birefringence and excellent optical properties, i.e., having linear transmittance and refractive index. Also, a translucent ceramic including a Ba(Zn, Ta)O3 type perovskite as a main component is disclosed in Patent Document 4.
Optical properties such as refractive indices and Abbe numbers of the translucent ceramic described in Patent Document 3 can be changed by substituting a part of Mg and/or Ta with Sn and/or Zr.
Similarly, the translucent ceramic disclosed in Patent Document 4 can be also changed in refractive index and Abbe number.
However, since the translucent ceramics described in Patent Document 3and Patent Document 4 have small Abbe numbers, applications as optical components or optical apparatuses are limited.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-127078 (all pages and FIG. 1)    Patent Document 2: Japanese Unexamined Patent Application Publication No. 7-244865 (claim 6 and paragraph [0024])    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-75512 (all pages and all drawings)    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2004-75516 (all pages and all drawings)